The former British spy identified in media reports as the author of a dossier of allegations against Donald Trump is a reliable and thorough intelligence specialist, a retired top U.K diplomat said.

Christopher Steele, who worked for MI6 and now runs a private intelligence firm, would not invent evidence, Andrew Wood, Britain's ex-ambassador to Russia, told BBC Radio on Friday.

"I know him as a very competent, professional operator who left the secret service and is now operating his own private company," said Wood, who served as U.K. envoy in Moscow from 1995 to 2000. "I don't think he would make things up."

The U.K. government has declined to comment on Steele’s involvement in the controversy over the release of a dossier of unsubstantiated allegations about Trump’s personal life and business activities. Wood is the most senior British figure so far to vouch for the reliability of Steele’s work. The president elect has described the claims as “fake news” and “phony stuff.”

Wood disclosed that he had discussed the dossier before it became public with Senator John McCain, who then made efforts to obtain his own copy.

He said it is not surprising that Steele had gone into hiding, as several newspapers have reported, after he was named as the author of the dossier. “If his sources have told him the truth and he has conveyed the truth, that in itself is rather dangerous knowledge,” he said. “Russia would certainly like to know where he has got his information from” and Moscow is “accustomed to take action,” he added.

The former British spy identified in media reports as the author of a dossier of allegations against Donald Trump is a reliable and thorough intelligence specialist, a retired top U.K. diplomat said. Christopher Steele, who worked for MI6 and now runs a private intelligence...